1. Field of the Invention
The present invention relates to a semiconductor laser device with a dielectric multilayer film.
2. Description of the Related Art
Generally, a semiconductor laser has resonator end faces formed by wafer cleavage, and dielectric films are formed on the resonator end faces. A desired reflectance of each end face can be controlled by arbitrarily selecting species, film thickness and number of layers of the dielectric film on the end face.
Such a reflective film requires not only characteristics of arbitrarily controllable reflectance but also of high tolerance to degrading due to catastrophic optical damage (COD). The COD degradation means that the film on the end face of a laser device is heated up by absorbing laser light and then melted down as temperature rises, resulting in destruction of the end face.
When forming a reflective film with 40% or more of reflectance, for example, a dielectric multilayer film with a low refractive index film and a high refractive index film laminated alternately is employed in general. The related prior arts are listed as follows:
[Document 1] Japanese Patent Unexamined Publications (koukai): JP-H10-247756 (1998), A
[Document 2] Japanese Patent Unexamined Publications (koukai): JP-2001-267677, A
[Document 3] Japanese Patent Unexamined Publications (koukai): JP-2002-305348, A
For example, the document 2 (JP-2001-267677) employs a multilayer reflective film of five layers including a Al2O3 film and a Si film containing oxygen for the high reflective film on the rear end face of a semiconductor laser, in which introduction of oxygen into a deposition process of Si film enables an extinction coefficient of Si to decrease, thereby preventing the COD degradation. However, with laser oscillation wavelength shortened and laser power heightened, the Si film is likely to have an optical absorption coefficient, which may exceed a certain limit to cause the COD degradation.
Meanwhile, the document 1 (JP-H10-247756) employs a multilayer film of titanium oxide (TiO2) and silicon oxide (SiO2) for the reflective film on the optical exit face of a semiconductor laser to improve the COD level. However, titanium oxide has low thermal stability in emission and is likely to age. Therefore, the reflectance may change because of variations of thickness and refractive index of the film, finally the COD degradation will occur.
Moreover, the document 3 (JP-2002-305348) employs a multilayer film of niobium oxide (Nb2O3) and silicon oxide (SiO2) for the reflective film on the end face of the resonator of a semiconductor laser with an oscillation wavelength of 400 nm.
In conventional semiconductor lasers, a multilayer reflective film including a high refractive index film, such as Si film, titanium oxide (TiO2) film, is studied. However, with laser power further heightened in the future, temperature of the laser end face will increasingly rise in emission. Therefore, the COD degradation and the aging, such as change of reflectance due to variations of thickness and refractive index, are concerned.